The present invention relates generally to the method for operating and the apparatus for dispensing a plurality of liquids through a single orifice, and more particularly, to the method and apparatus for dispensing and cleaning the chemical reactants required to form polyurethane foams from the dispensing assembly.
Polyurethane foams are formed by the reaction of an isocyanate component A and hydroxyl-bearing compounds. When mixed in the presence of a catalyst and other additives, such as a polyether resin, a surfactant, a catalyst, and a blowing agent, these chemicals react to form cross-linked polymer chains, more commonly known as a polyurethane. Each of these components of the plural component material, by itself, is generally stable. Thus, each component will not cure or cross-link for extended periods of time, often as long as several months, if they are properly stored. However, when the isocyanate component A and the chemical polyol component B, a preformulated compound formed from the aforementioned resin, surfactant, catalyst and blowing agent, are mixed together in proper concentrations, an exothermic chemical reaction of the isocyanate and polyol occurs causing a continued expansion that is evidence of the polymerization and manifests itself as foam which cross-links and cures. The cross-linking and curing usually is substantially completed in a matter of seconds.
Polyurethane foam dispensers are well known and have achieved a high degree of usage in factories where components must be adhesively lined with an insulating foam or where products must be packaged and protected from damage during shipment. This high level of use of polyurethane foam dispensing equipment has also focused attention on the efficiency of prior apparatus employed to accomplish this dispensing. Many of the problems arise from the fact that the polyurethane foam "sets" or builds up in the dispenser, normally within the mixing chamber or the nozzle, after the chemical components have cross-linked and begun to cure. This "setting" can eventually cause the apparatus to become inoperative due to plugging. In operations such as those required during packaging where intermittent use of the dispensing apparatus is required, the "setting" problem is more severe. This typically occurs in the situation where a packer initially directs a "shot" of the mixed plural components into the bottom of a container, inserts a polyethylene strip over the top, and places the product to be shipped in the container. Another sheet of polyethylene is placed on top of the product, and the foam dispensing apparatus is then activated after a delay of 10 to 20 seconds from the time the first "shot" was dispensed to fill the box with the cushioning foam. This procedure is repeated for each item to be packed.
Prior foam dispensing apparatus has attempted to solve this "setting" problem by using either separately or in combination air blasts, or cleaning rods or plungers, or solvent to remove the residue foam from the dispensing assembly by scraping. Specific polyurethane foam systems have attempted to use air with pressurized solvent blown into the mixing chamber and the dispensing nozzle, an automatic solvent flush that runs through the mixing chamber and dispensing nozzle, a blast of purging gas preceding the continuous pumping of solvent through the mixing chamber and dispensing nozzle, and reciprocating cleaning rods or plungers which may or may not use the solvent to facilitate the scraping of residue foam from the dispensing apparatus.
All of the systems have suffered from the disadvantage of employing moving parts which cause additional wear and tear, such as the cleaning rods, or have required active solvent flush systems whereby the solvent was either continuously recycled through the mixing chamber and dispensing nozzle or periodically blown into the mixing chamber and dispensing nozzle under pressure. The use of a continuous stream of solvent or the intermittent delivery of solvent under pressure required additional pumps and solvent reservoirs within the apparatus that are costly and conducive to untimely breakdowns and repairs. Additionally, the prior foam dispensing apparatus has generally required the actual dispensing assembly to be broken down frequently to permit at least the mixing chamber and the discharge nozzle to be cleaned of the set foam residue.
These problems are solved in the design of the present invention by providing a method and apparatus for dispensing plural component polyurethane foam from a dispensing assembly by employing a combination of air purge and immersion of a portion of the assembly in a static solvent bath.